Variation in the state of electrical discharge of the load may cause fluctuation of impedance. For example, the impedance may fluctuate between before starting discharge and after starting discharge in the plasma load. Also in the course of discharging, the impedance may vary due to the state of gas supplied to the plasma, creations within a plasma chamber, or environmental changes.
It is known that impedance matching is performed by adjusting a matching circuit that is provided between the RF power supply device and the load, so as to supply power efficiently from the RF power supply device to the load. It has been pointed out that the impedance matching according to this matching circuit has a problem, since it is difficult to respond to abrupt impedance variation.
To address this problem, utilizing a feature that an output frequency from an RF power supply device is allowed to be variable instantly, there has been suggested the RF power supply device that changes an output frequency and thereby varying the impedance on the load side, so as to perform impedance matching instantaneously (patent document 1). It has also been suggested that frequency control of the output frequency is performed by changing the frequency, using the absolute value of reflection coefficient as an index (patent documents 1 and 2).
It has been suggested in the patent document 1, a processing is repeated for specifying an oscillation frequency that minimizes the absolute value of the reflection coefficient, along with varying the oscillation frequency within a predetermined frequency range including a reference frequency, thereby controlling the oscillation frequency so that the absolute value of the reflection coefficient becomes smaller.
It has been suggested in the patent document 2, frequency sweep is performed within a defined frequency range, in a constant cycle, so as to detect a frequency that minimizes the reflected power.
It has further been suggested in the patent document 1, in the frequency control after the frequency is specified, there is defined an upper limit of the reflection coefficient in association with the absolute value of the reflection coefficient at that moment, and when the absolute value of the reflection coefficient exceeds the upper limit, the frequency control is repeated again for detecting the oscillation frequency that minimizes the absolute value of the reflection coefficient.